A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The imaging of the pattern (which typically includes micron or nanometer sized structures) can be very sensitive to dust or other contamination of the patterning device and/or the substrate. Therefore, before imaging, the patterning device (and/or a pellicle protecting the small structures thereof) and/or substrate may be tested for contamination, in particular for particles, using a contamination inspection system. If a particle is detected on the patterning device or on the substrate, the particle may be accepted (thereby accepting a possible fault area on the substrate), or it may be removed, or the patterning device or substrate may be rejected.